Plasmid DNA is separated by physical rather than chemical techniques. The chromosomal DNA consists of long strands that form sticky masses when cells are lysed. A standard prep uses SDS + NaOH for the lysis, followed by neutralization with KAc. The potassium dodecyl sulfate (KDS) which forms is insoluble, and precipitates out as a white solid, taking the stringy chromosomal DNA with it. The short plasmid DNA remains in solution and is separated for further purification. This is not a perfect process, and relies on users not shearing or vortexing the lysed cells. Even so, chromosomal DNA contamination is inevitable.

-phage434-

Hi,

Your lab must have the "Molecular Cloning" books.
Every lab has them. Or maybe "Current Protocols".
If you do not have them, get them and read them.
EVERY lab in the world has them.
They have great protocols for plasmid purification.
I don't recommend Cesium Chloride because it is time consuming.
I do recommend the Polyetylene Glycol method. Not as pure in general but much
easier.